The invention described herein was made by employee(s) of the United States government and may be manufactured and used by or for the Government of the United States of America for governmental purpose without payment of any royalties thereon or therefor.
1. Field of the Invention
The present invention relates generally to signal tracking systems and, more specifically, to a passive system for tracking/locating a transmitter.
2. Background of the Invention
Determining the location and/or tracking a moveable transmitter is useful for many purposes. For instance, there frequently exists an urgent need for locating the source of an emergency 911 telephone call from a mobile phone. Previous options for this application included utilizing GPS location devices but these require additional circuitry, additional antennas, do not necessarily operate indoors, and may be slow to obtain a location fix.
Other tracking requirements might include tracking an astronaut in space during extravehicular activity. Another desirable use might be to track small vehicles in space such as, for instance, a small one-foot diameter autonomous vehicle for flying anywhere around the Space Station and transmitting video from an onboard camera.
It is also increasingly important to be able to locate, track, communicate with or activate vehicle systems wirelessly, and/or otherwise monitor fleets or individual trucks, automobiles, containers, and other moveable targets. Such monitoring is increasingly utilized and has, for instance, become a standard feature on many automobiles. Previous prior art options to perform this function typically include utilizing GPS location devices with the same problems as listed above. It would be very useful to somehow provide such monitoring in a low cost, reliable, high-speed manner that avoids/alleviates the problems associated with presently existing GPS location systems.
The above are only a few specific uses, and it will be understood that a small, inexpensive system that is capable of passively tracking a transmitter by utilizing the transmitter""s data modulated signal, and which may operate in a noisy environment with mulitpath signals, may be useful for a myriad of applications and systems.
Tracking objects by receiving signals therefrom is well known. For instance, radar has been utilized for most of the last century to track objects. However radar is nonpassive, requires high power, and also requires a recognizable signature at the target for identification.
Various types of signal processing for estimation of signal parameters have been used as far back as 1795. A more recent signal processing method is that of ESPRIT (Estimation of Signal Parameters using Rotational Invariance Techniques) which is discussed in at least one of the subsequently listed patents. However, the ESPRIT technique is based on relatively complicated mathematics. The inventors believe that using the ESPRIT technique for the purposes of the present invention may require at least four antennas, the generation of complex matrices, the calculation of complex eigenvalues and eigenvectors, and the estimation of noise level. The complexity of these requirements effectively renders the ESPRIT technique unacceptable for many applications requiring low weight and power and small size.
It might also be noted that techniques exist for locating the source of a magnetic field but such techniques require a powerful magnetic pulse generator at the source.
The following patents show attempts to solve problems related to the present invention but do not show the solution provided by the present invention:
U.S. Pat. No. 5,987,016, issued Nov. 16, 1999, to R. He, discloses a method for tracking a mobile communication signal, which operates in a code division multiple access wireless communication system including an antenna, and a base site receiver having at least two receiver tracking fingers, includes receiving at the antenna a first multipath signal arriving at an on-time pn-offset with an associated advanced pn-offset value and retard pn-offset value and receiving at the antenna a second multipath signal arriving at an on-time pn-offset with an associated advanced pn-offset value and retard pn-offset value. The method further includes determining a spacing between the first multipath signal and the second multipath signal, and adjusting the at least two receiver tracking fingers based on the advanced pn-offset value of one of the multipath signals and the retard pn-offset value of the other multipath signal.
U.S. Pat. No. 5,691,974, issued Nov. 25, 1997, to Zehavi et al., discloses a method and apparatus for tracking the frequency and phase of signals in spread spectrum communication systems that makes more efficient use of available carrier frequency and phase information by utilizing a substantial portion or all of the energy occupying the frequency spectrum of a received carrier signal, including energy from communication signals intended for other system users. Multiple spread spectrum communication signals are input in parallel to data receivers where they are despread using preselected despreading codes at an adjustable phase angle and decoded over multiple orthogonal codes active within the communication system. Multiple decoded signals are then combined to form a single phase detection signal which is used by at least one tracking loop to track frequency and phase of the carrier signal for the received communication signals. The tracking loop generates a timing signal which is used to adjust the phase angle used during despreading. In further embodiments, the communication signals are despread using appropriate PN codes and separated into in-phase (I) and quadrature channels (Q) where data symbols are processed by fast Hadamard transformers to generate corresponding data bits. The data is formed into pairwise products between the channels and summed over multiple or all active subscriber orthogonal codes. This sum indicates a degree to which the estimated phase differs from the actual phase of received communication signals and is used to adjust the phase of application for the PN codes.
U.S. Pat. No. 5,621,752, issued Apr. 15, 1997, to Antonio et al., discloses a system and method for adaptively sectorizing channel resources within a digital cellular communication system is disclosed herein. The system includes an antenna arrangement for providing at least first and second electromagnetic beams for receiving a first information signal transmitted by a specific one of a plurality of users, thereby generating first and second received signals. A first set of beam-forming signals are then generated from the first and second received signals. A demodulating receiver is provided for demodulating at least first and second beam-forming signals included within the first set of beam-forming signals, thereby producing first and second demodulated signals. The system further includes a tracking network for tracking multipath information signals, received from various positions and angles of incidence, based on comparison of the first and second demodulated signals.
U.S. Pat. No. 6,147,641, issued Nov. 14, 2000, to J. Issler, discloses a process for the autonomous reduction of acquisition and tracking thresholds of carriers received in orbit by a receiver accessing an orbital navigator inside or outside said receiver, the latter having at least one phase loop. The phase loop, which is responsible for the acquisition and/or tracking of the carrier, is xe2x80x9cpushedxe2x80x9d by the fine speed aid and takes up the error between the real speed and the calculated speed. The search for the Doppler frequency of the carrier received takes place around a frequency prediction maintained by the fine speed aid coming from the orbital navigator.
U.S. Pat. No. 5,914,949, issued Jun. 22, 1999, to G. Y. Li, discloses a finger tracking circuit for a rake receiver, a method of tracking a carrier signal and a wireless infrastructure. The finger tracking circuit includes: (1) a timing error subcircuit that determines a timing error in a current power control group (xe2x80x9cPCGxe2x80x9d) of a carrier signal to be tracked and (2) a feedback subcircuit that applies a gain signal that is a function of a data rate of the carrier signal and a signal-to-noise ratio (xe2x80x9cSNRxe2x80x9d) to the timing error subcircuit, a convergence rate of the finger tracking circuit therefore depending on the data rate of the carrier signal.
U.S. Pat. No. 5,859,612, issued Jan. 12, 1999, to K. S. Gilhousen, discloses a method for determining the position of a mobile station within a cellular telephone system having a plurality of base stations. A signal is transmitted from a rotating antenna. The rotating antenna has a beam which rotates around a cell in the cellular telephone system. The beam has a rotational timing that is known by the mobile station. The signal is received at the mobile station. Based on a reception time when the signal is received by the mobile station, an angular displacement value corresponding to the position of the mobile station is determined. A first round trip signal propagation time between a stationary antenna and the mobile station is measured using a voice information signal. The position of the mobile station is determined in accordance with the angular displacement value and the first round trip signal propagation time. A method for determining the position of a mobile station within a cellular telephone system having a plurality of base stations. A voice information signal is transmitted from the mobile station. The voice information signal is received with a first antenna having a rotating beam for receiving the signal. Based on a reception time when the voice information signal is received by the first antenna, an angular displacement value corresponding to the position of the mobile station is determined. A first round trip signal propagation time between a second antenna and the mobile station is measured. The position of the mobile station is determined in accordance with the angular displacement value and the first round trip signal propagation time.
U.S. Pat. No. 5,602,833, issued Feb. 11, 1997, to E. Zehavi, discloses a method and apparatus for generating orthogonally encoded communication signals for communication system subscribers using multiple orthogonal functions for each orthogonal communication channel. Digital data symbols for signal recipients are M-ary modulated using at least two n-length orthogonal modulation symbols, which are generally Walsh functions normally used within the communication system. These symbols are provided by a modulation symbol selector typically from one or more code generators, and the modulation is such that M equals a product of a total number of orthogonal functions and the number used to generate individual modulation symbols. Each group of log M encoded data symbols from data processing elements are mapped into one modulation symbol using the modulation symbol selection element according to their binary values. In some embodiments, a fast Hadamard transformer is used for symbol mapping. The resulting communication signals are demodulated by correlating them with the preselected number of orthogonal functions, in parallel, and demodulating the results into M energy values representing each orthogonal modulation symbol. The energy values are mapped into energy metric data using a dual maximum metric generation process. The correlation and demodulation can be accomplished using at least two sets of N correlators, N being the number of functions used, and applying correlated signals to one demodulator for each set of correlators. Each demodulator outputs M energy values representing each of the M mutually orthogonal modulation symbols, which are then combined into a single set of M energy values. In further configurations, coherent demodulators can be used to produce amplitude values for received signals which are then combined with dual maximum metric results to produce composite metric values for data symbols.
U.S. Pat. No. 4,750,147, issued Jun. 7, 1988, to Roy, III, et al., discloses an invention relating generally to the field of signal processing for signal reception and parameter estimation. The invention has many applications such as frequency estimation and filtering, and array data processing, etc. For convenience, only applications of this invention to sensor array processing are described herein. The array processing problem addressed is that of signal parameter and waveform estimation utilizing data collected by an array of sensors. Unique to this invention is that the sensor array geometry and individual sensor characteristics need not be known. Also, the invention provides substantial advantages in computations and storage over prior methods. However, the sensors must occur in pairs such that the paired elements are identical except for a displacement which is the same for all pairs. These element pairs define two subarrays which are identical except for a fixed known displacement. The signals must also have a particular structure which in direction-of-arrival estimation applications manifests itself in the requirement that the wavefronts impinging on the sensor array be planar. Once the number of signals and their parameters are estimated, the array configuration can be determined and the signals individually extracted. The invention is applicable in the context of array data processing to a number of areas including cellular mobile communications, space antennas, sonobuoys, towed arrays of acoustic sensors, and structural analysis.
The above prior art does not disclose a system that uses a herein disclosed technique in one preferred embodiment for extracting a direction vector from a pair of phase shifts. Moreover, the prior art does not disclose the herein disclosed technique for extracting the phase difference between a transmitter and receiver, which in one preferred embodiment may provide a noncoherent demodulation scheme, and then using the phase difference to detect the location of the transmitter.
Therefore, those skilled in the art have long sought and will appreciate the present invention that addresses these and other problems.
An object of the present invention is to provide an improved transmitter tracking system and method.
Yet another object of the present invention is to provide an improved system and method for locating and/or tracking the transmitter of a data-modulated electromagnetic signal that is operable in the presence of noise and/or multipath interference.
One of many advantages of the present invention is that the system does not require a coherent reference at the receiver.
One of many features of a preferred embodiment of the present invention is a novel technique for extracting the relative phase difference between a transmitter and a receiver.
Another of many features of a preferred embodiment of the present invention is that the system and method may convert the phase difference between a transmitter and receiver into a unit direction vector.
An advantage of the present invention is an inexpensive, quickly operating system that can be utilized with any communication system for locating a moveable transmitter.
These and other objects, features, and advantages of the present invention will become apparent from the drawings, the descriptions given herein, and the appended claims. It will be understood that above listed objects, features, and advantages of the invention are intended only as an aid in understanding aspects of the invention, are not intended to limit the invention in any way, and do not form a comprehensive list of such objects, features, and advantages.
Therefore, the present invention comprises a passive system for locating a transmitter comprising one or more elements, such as for instance, at least one antenna array comprising a first antenna element, a second antenna element and a third antenna element. The first antenna element is operable for receiving a first received signal from the transmitter, the second antenna element is operable for receiving a second received signal from the transmitter, and the third antenna is operable for receiving a third signal from the transmitter. Other elements may include electronic circuitry to determine a first phase difference between the first received signal and the second received signal as well as a second phase difference between the first received signal and the third received signal. The electronic circuitry is preferably operable for determining an orientation of a vector from the antenna array to the transmitter by utilizing the first phase difference and the second phase difference.
In a preferred embodiment, the second antenna element and the third antenna element may be spaced apart from the first antenna element by one-half wavelength or integer multiple thereof. A preferred embodiment antenna array comprises only three antenna elements consisting of the first antenna element, the second antenna element, and the third antenna element. A geometrical configuration is provided for the first antenna element, the second antenna element, and the third antenna element such that a first leg between the first antenna element and the second antenna element and a second leg between the first antenna element and the third antenna element have a first angle therebetween less than one hundred eighty degrees. Preferably, the first angle is ninety degrees for efficient operation. Preferably, the first antenna element, the second antenna element, and the third antenna element each comprise a microstrip patch antenna.
The electronic circuitry may further comprise a local oscillator operating at the same frequency as the transmitter but not phase locked with respect to the transmitter frequency. The electronic circuitry may further comprise a spread spectrum receiver with a first receiver channel for processing the first received signal from the first antenna element, a second receiver channel for processing the second received signal from the second antenna element, and a third receiver channel for processing the third received signal from the third antenna element. Additional electronic circuitry may include a first downconverter for the first receiver channel, a second downconverter for the second receiver channel, and a third downconverter for the third receiver channel. Furthermore, the electronic circuitry may comprise a first finger for the first receiver channel, a second finger for the second receiver channel, and a third finger for the third receiver channel. In a preferred embodiment, each of the first finger, the second finger, and the third finger are operable for performing a Fast Walsh Transform to determine a winning Walsh symbol based on magnitude and not phase of a Walsh vector.
In accord with a preferred embodiment of the invention, a method is provided for passively detecting the location of the transmitter comprising one or more steps, such as for instance, receiving the transmitter signal with a first antenna array comprising a first antenna element that produces a first received signal, a second antenna element that produces a second received signal, and a third antenna element that produces a third received antenna signal. Other steps may include determining a first phase difference between the first received signal and the second received signal, determining a second phase difference between the first received signal and the third received signal, and utilizing the first phase difference and the second phase difference to determine a first vector in the direction of the transmitter from the first antenna array.
In one embodiment, the method may include receiving the transmitter signal with a second antenna array spaced from the first antenna array, the second antenna array comprising one or more elements, such as for instance, a fourth antenna element that produces a fourth received signal, a fifth antenna element that produces a fifth received signal, and a sixth antenna element that produces a sixth received antenna signal, determining a third phase difference between the fourth received signal and the fifth received signal, determining a fourth phase difference between the fourth received signal and the sixth received signal, and utilizing the third phase difference and the fourth phase difference to determine a second vector in the direction of the transmitter from the second antenna array. Additional steps may further comprise utilizing the first vector and the second vector for locating the transmitter.
The method may also comprise providing a local oscillator with the same frequency as the transmitter frequency but not phase locked with respect to the transmitter frequency and/or processing the first received signal, the second received signal, and the third received signal in a spread spectrum receiver. The spread spectrum receiver may perform steps such as downconverting and despreading the first received signal, the second received signal and the third received signal.
In a preferred embodiment for use with multipath noise, the method may comprise tracking multiple transmitter paths of the first received signal, the second received signal, and the third received signal. The tracking may comprise separately time multiplexing the multiple transmitter paths for each of the first received signal, the second received signal, and the third received signal. Other steps may comprise indexing multipath components for the first received signal, the second received signal, and the third received signal with respect to a position of each of the multipath components with respect to a generated local PN sequence. Preferably the method includes steps such as comparing an indexed multipath signal of the first received signal to a corresponding indexed multipath signal of the second received signal and the third received signal to produce a multipath comparison and/or utilizing the multipath comparison to determine the first phase difference and the second phase difference.
Operational steps may include storing a plurality of modulation symbols, performing a Fast Walsh Transform on the plurality of modulation symbols to determine a winning symbol, comparing the winning signal to the plurality of symbols to determine a signal to noise ratio, and utilizing the signal to noise ratio to determine whether a local PN-generator is aligned with the transmitted signal.
In other words, the method may comprise providing a noncoherent receiver whereby a local oscillator has frequency equal to the transmitter frequency but not phase locked with respect to the transmitter frequency, determining a phase difference between the transmitter and the noncoherent receiver, and utilizing the first phase difference to produce a vector which points in the direction of the transmitter.
In a preferred embodiment the method comprises steps such as providing that the receiver is a spread spectrum receiver operable for orthogonal symbol modulation, detecting bits at any arbitrary angle between the transmitter and receiver, obtaining a plurality of modulation symbols, and selecting a winning modulation symbol from the plurality of modulation symbols based solely on magnitude related to the winning modulation symbol. Further steps may include utilizing a phase related to the winning modulation symbol to represent a phase difference between the transmitter and the noncoherent receiver such as the a phase difference related to the first, second, or third received signals.
Restated, the method may comprise steps such as receiving a plurality of modulation symbols with a receiver, determining a magnitude and phase for each of the modulation symbols, comparing the plurality of modulation symbols with respect to their magnitudes to determine a winning modulation symbol from the plurality of modulation symbols, and utilizing a phase of winning symbol for representing a relative phase between the transmitter and the receiver.
Additional steps may comprise producing a local PN code signal which corresponds to the transmitted PN code signal of the transmitter, producing a floor from the magnitudes of the plurality of modulation symbols, comparing the floor with a magnitude of the winning symbol to produce a value related to signal to noise, and utilizing the value to determine if the local PN signal is aligned with the transmitted PN code signal of the transmitter. If the value is outside of a desired range, then the method may include one or more steps of shifting the local PN signal. If the value is within a desired range, then the method may comprise utilizing the phase of the winning symbol to determine early and late modulation symbols. The method may further comprise comparing the early and late modulation symbols to previously stored early and late modulation signals.
The present invention also comprises a method for determining a phase between a spread spectrum transmitter and spread spectrum receiver comprising one or more steps, such as for instance, receiving a plurality of modulation symbols with the receiver, determining a magnitude and phase for each of the modulation symbols, comparing the plurality of modulation symbols with respect to the magnitude to determine a winning modulation symbol from the plurality of modulation symbols, and utilizing a phase of winning symbol for representing a relative phase between the transmitter and the receiver.
A method for a passive system in accord with the present invention may be provided for determining location characteristics of a plurality of moveable transmitters, wherein each of the plurality of moveable transmitters preferably produces a transmitter signal, comprising steps such as providing a plurality of receivers spaced apart wherein each of the plurality of moveable transmitters is receivable by at least one of the plurality of receivers, providing each receiver with an antenna array preferably with three spaced apart antenna elements, determining a pair of transmitter signal phase shifts at the three spaced apart antenna elements for a respective first moveable transmitter and first receiver, and utilizing the pair of transmitter signal phase shifts to determine a first direction of the first moveable transmitter with respect to the first receiver.
In one operational mode the method comprises utilizing a receiver generated PN signal to determine a distance from the first receiver to the first moveable transmitter, and utilizing the distance and the first direction to determine a position of the first moveable transmitter.
In another operational mode, the method comprises determining a second pair of transmitter signal phase shifts at a second of the three spaced apart antenna elements for the respective first moveable transmitter and a second receiver, utilizing the second pair of transmitter signal phase shifts to determine a second direction of the first moveable transmitter with respect to the second receiver, and utilizing the first direction and the second direction to determine a position of the first moveable transmitter.
In another operation mode, the method comprises obtaining a possible path of travel of the first moveable transmitter, and utilizing the first direction and the possible path of travel for determining a position of the first moveable transmitter.
Other steps may include displaying a position of one or more of the plurality of moveable transmitters on a map and/or displaying the map in a vehicle to which the moveable transmitter is affixed.